Recently, with the advent of a highly information-based society, materials satisfying both heat resistance and transparency have become desired in the field of optical communications including optical fibers, optical waveguides, etc., and in the field of display devices including liquid-crystal orientation films, color filters, etc.
In the field of display devices, an alternative technology of employing plastic substrates that are lightweight and are excellent in flexibility, in place of glass substrates, and a development of displays capable of being bent or rolled up are now under way. However, for example, when an electronic element comprising an inorganic material is formed on a film, then the film having the inorganic element formed thereon may bend and, as the case may be, the inorganic element may often peel away from the film, since the inorganic material and the film significantly differ in point of the linear expansion coefficient. Accordingly, it is desired to develop a resin material for films having both transparency and heat resistance and having a low thermal linear expansion coefficient.
Polyimide has excellent heat resistance and additionally has other excellent properties of mechanical characteristics, chemical resistance, electric characteristics and the like, and therefore films formed of a material of polyimide are widely used in various fields of molding materials, composite materials, electric and electronic components, display devices, etc. However, those films are further required to have higher transparency and dimensional stability than ever.
In general, it is known that polyimides having the polymer chains which are more rigid and have a higher linearity have a lower thermal linear expansion coefficient, and for improving the dimensional stability of polyimides by lowering the thermal linear expansion coefficient thereof, various structures of both acid dianhydrides and diamines that are source materials of polyimides have heretofore been proposed.
PTL 1 discloses a polyimide film containing a specific repeating unit, which is excellent in heat resistance and linear expansion coefficient. However, the polyimide film disclosed in PTL 1 is strong in yellow, that is, it has a Y1 value of around 15 when having a thickness of 50 μm, and therefore, for realizing sufficient transparency, it is desired to further reduce the coloration of the film.
PTL 2 discloses a polyamic acid resin composition containing from 0.3 to 15% by weight of a nanolayer silica sheet and/or a nanometer silica powder, and a polyimide film formed of the composition and having characteristics of low water absorbability, high transparency and high dimensional stability. However, the polyimide film disclosed in PTL 2 has a transmittance of from 30 to 40% when having a thickness of 25 μm, and depending on the intended use thereof, the transparency of the film would have to be further increased. In addition, when the silica content is increased for enhancing the heat resistance and the dimensional stability, the transparency of the film is expected to lower.
PTL 3 discloses a polyimide resin produced by reacting a 1,2,4,5-cyclohexanetetracarboxylic acid derivative component with a diamine component having a specific skeleton, a polyimide varnish containing the polyamide resin and silica microparticles, and a polyimide molded product excellent in transparency and flexibility, which is produced by molding it. However, the polyimide molded product disclosed in PTL 3 is excellent in transparency but is problematic in that the thermal linear expansion coefficient is high.
PTL 4 discloses a polyimide film mainly containing silicon dioxide microparticles having a specific particle size and a polyimide. However, the films disclosed in Examples in PTL 4 have a light transmittance of 90% when having a thickness of about 13 μm, and in addition, the mean thermal expansion coefficient thereof is more than 50 ppm, and therefore, the film is desired to be improved in point of both the transparency and the thermal expansion coefficient thereof.